This invention relates generally to high-power lasers and, more particularly, to free-electron lasers. A free-electron laser produces a small-diameter, parallel, diffraction-limited, optical beam of such high intensity that there is a practical difficulty in focusing and reflecting the beam.
By way of background, a free-electron laser generates coherent light when bunches of free electrons, accelerated to near relativistic velocities, i.e. near light speed, are passed through a spatially alternating magnetic field known as a wiggler. Considerable energy is required to accelerate the electrons to near light speed, and the overall efficiency of such a system is typically only a few percent. Basically, a free-electron laser has two principal components: the laser itself, in which some of the energy of the electrons is converted into light energy, and an electron acceleration system. The present invention is not directly concerned with either of these components, but relates to improvements in practical techniques for handling a high-power beam outcoupled from such a laser, or from multiple lasers of the same type.
Optical lenses, mirrors and windows made from conventional materials are subject to damage or destruction from a high-power free-electron laser (FEL) beam. A common solution to this problem is to locate optical mirrors a large distance from the laser gain region from which the beam is emitted. Over a long distance, perhaps tens or hundreds of meters, the output beam diffracts, broadens, and loses intensity. Location of the optical components at such large physical spacings poses significant problems of beam alignment and mechanical design. The problem is aggravated by the presence, in the beam, of output components at harmonic frequencies in addition to the fundamental frequency. Although their initial intensities are less than that of the fundamental component, these harmonic components may have a total power of about one percent of the total beam power. Moreover, the higher harmonics suffer less diffraction and can still cause serious damage to remotely located optical components.
The interaction between relativistic electrons and the alternating magnetic field takes place in a vacuum in the FEL. Consequently, the first problem facing the designer of a FEL system is how to seal this vacuum and provide a window for output of the outcoupled beam. A window of conventional optical materials positioned close to the laser gain region would be destroyed in a high-power system. The solution commonly proposed is to contain the entire optical system within a vacuum until the beam is sufficiently broadened to permit reflection by suitably cooled mirrors and windows.
There are, therefore, two basic problems associated with FELs of high power. The first is that there has been no practical technique for conveniently handling high-power output beams from FELs without the use of very widely spaced optical components. The second, and related problem is that harmonic components of the laser output are even more difficult to handle with conventional optics, since they are less subject to diffraction. The present invention is directed to a solution to both of these problems. A third category of problems relates to the correction of laser beams for wavefront distortions caused by passage through optical components or through a transmission medium. Although solutions have been proposed using phase conjugation techniques for this purpose, there is an inherent difficulty in applying these techniques to FEL beams, because of their extremely high power. The present invention permits single or multiple beams to be conveniently corrected for wavefront distortions caused by passage of the beam through optical components or through a transmission medium. A further advantage is that medical applications of FELs are greatly enhanced by the compact nature of the optical system of the invention, and by the capability of separately outcoupling harmonics of the FEL fundamental.